Flush-Then-Fill Planning for Glycol and Heat-Transfer Fluids

Why the fill changes the flush plan

Most mechanical contractors plan a hydronic flush and a fluid fill as two separate line items purchased from two separate subcontractors on two separate schedules. That approach works when the loop will be filled with treated water. When the specification calls for a glycol solution or a manufacturer-specified heat-transfer fluid, it stops working.

The reason is that the flush and the fill share a common boundary: the condition of the system at the moment the fluid enters. Residual flush water changes the final concentration. A system that was not fully drained and dried before filling starts out at a lower glycol percentage than calculated, and the only way to correct it is to remove fluid and add more, which costs money and time that nobody planned for. The answer is to plan the flush and the fill together from the beginning, during preconstruction, before the mechanical subcontract is priced.

Calculating the volume you will actually buy

Fluid volume is calculated from system volume, not from a rule of thumb. System volume comes from pipe-size data: diameter, length, and the volume of major equipment such as chillers, cooling towers, heat exchangers, and expansion tanks. The calculation needs to go loop by loop because different parts of the system may have different fluid specifications.

The calculated system volume is the starting point. Actual fluid purchases are higher because of transfer losses, the volume needed to circulate and verify concentration, and any allowance for makeup fluid during commissioning. Premixed fluid also comes in drums or totes that do not subdivide neatly, so there is typically a rounding increment to account for. Planning for all of these during preconstruction produces a purchase quantity that does not surprise anyone when the delivery invoice arrives.

Residual water and why concentration drifts

After a hydronic flush, the system is never completely empty. Water stays in low points, dead legs, equipment sumps, and low-point drains that were not opened during demobilization. When glycol or a heat-transfer fluid is added to a system that still contains residual flush water, the residual water dilutes the fluid and the final concentration is lower than calculated.

How much lower depends on system geometry. A system with many low points and long dead legs retains more water than a compact system with well-placed drains. The fill plan should account for expected residual volume by targeting a slightly higher initial concentration, or by including a circulation and check step before declaring the fill complete. The engineer's or fluid manufacturer's acceptance criteria determine the target range; the fill plan determines how to hit it.

Premixed versus field-adjusted fluid

Fluid arrives either premixed to the target concentration or as concentrate that is diluted on site. Premixed fluid is simpler to receive and stage: the concentration is fixed, lot documentation is straightforward, and there is no mixing step that can go wrong. The tradeoff is that premixed fluid is heavier to ship and less flexible if the target concentration changes.

Field-adjusted fluid requires mixing water and concentrate in the correct ratio before transfer. The water source matters: the fluid manufacturer's requirements specify whether potable water, deionized water, or another source is acceptable. Mixing equipment, a way to measure concentration before the batch enters the system, and documentation of the mix ratio for each batch are all part of the field procedure.

For specialty products such as DOWTHERM or THERMINOL products when specified, premixed is typically the practical choice for large system charges because on-site mixing of high-temperature heat-transfer fluids introduces handling and documentation complexity that most project teams prefer to avoid.

Receiving, staging and traceability

Fluid arrives on a truck. Someone needs to be there to receive it, check the delivery against the purchase order, record lot and batch numbers, and stage the containers where they can be accessed by transfer equipment without blocking the site. None of this happens automatically, and all of it needs to be planned before the truck arrives.

Lot and batch documentation is the beginning of the traceability record. The lot number on the delivery ticket links to the fluid manufacturer's certificate of conformance, which confirms the product, concentration, and any quality data relevant to the specification. That documentation travels with the closeout package. If the specification requires sampling or third-party verification of the fluid before it enters the system, the sample plan needs to be in place before delivery.

Containment is a separate planning item. Fluid drums and totes need secondary containment during storage and transfer. Transfer equipment, including pumps, hoses, and fittings, needs to be compatible with the fluid. The site needs a designated staging area with access for the delivery vehicle and for the transfer setup. These requirements are typically not in the mechanical contract scope of work and need to be resolved in advance.

Verifying concentration at representative points

A concentration reading at the fill connection is a starting point, not a completion check. After circulation, concentration needs to be verified at representative points across the system: at least one high point, one low point, and one point distant from the fill connection. The number and location of sample points is typically defined by the specification or the engineer's requirements.

Refractometer readings are the standard field tool. The refractometer needs to be calibrated and appropriate for the specific fluid. Some specifications require laboratory analysis in addition to field readings, in which case chain-of-custody sample handling needs to be planned alongside the field verification step.

What the turnover record should contain

The fluid fill generates a set of records that belong in the commissioning closeout package. At minimum, the record should contain: the fluid product name and specification; delivery date; lot and batch numbers; certificate of conformance from the fluid manufacturer; calculated system volume by loop; fluid quantity received; fluid quantity transferred; any field-mix ratio records if applicable; concentration readings with date, location, and instrument identification; volume reconciliation showing calculated versus actual; and the name of the technician who performed each step.

If the engineer or commissioning authority requires a specific report format, that format should be identified during preconstruction. It is easier to collect field data in the right format the first time than to reconstruct it from incomplete notes after closeout.

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Project documents, the equipment manufacturer's requirements and the responsible engineer's direction govern on any specific project. This article is general planning guidance for contractors, not engineering advice.

For more on glycol and heat-transfer fluid filling as part of a coordinated flush scope, see Glycol & Heat-Transfer Fluid Filling. To send project documents or request a scope review, visit our contact page.